10. The wind turbine blade recited in claim 1, wherein the permeable flap
extends from a trailing edge of the blade between approximately 1% and 5%
of a chord of the blade.

11. The wind turbine blade recited in claim 10, wherein the permeable flap
extends from a trailing edge of the blade between approximately 2% and 4%
of a chord of the blade.

12. The wind turbine blade recited in claim 1, wherein the permeable flap
has a thickness of less than about 0.5% of a chord of the blade.

13. The wind turbine blade recited in claim 12, wherein the permeable flap
has a thickness of less than about 0.3% of a chord of the blade.

14. A wind turbine blade, comprising:a substantially flexible, permeable
flap extending from a trailing edge of the blade between approximately 1%
and 5% of a chord of the blade; andwherein the permeable flap has a
thickness of less than about 0.5% of the blade.

15. The wind turbine blade recited in claim 14, wherein the permeable flap
extends from a trailing edge of the blade between approximately 2% and 4%
of a chord of the blade.

17. The wind turbine blade recited in claim 14, wherein the permeable flap
has a thickness of less than about 0.3% of a chord of the blade.

18. The wind turbine blade recited in claim 15, wherein the permeable flap
has a thickness of less than about 0.3% of a chord of the blade.

[0003]The subject matter described here generally relates to fluid
reaction surfaces with means moving working fluid deflecting working
member part during operation, and, more particularly, to wind turbines
blades having permeable acoustic flaps.

[0004]2. Related Art

[0005]A wind turbine is a machine for converting the kinetic energy in
wind into mechanical energy. If the mechanical energy is used directly by
the machinery, such as to pump water or to grind wheat, then the wind
turbine may be referred to as a windmill. Similarly, if the mechanical
energy is converted to electricity, then the machine may also be referred
to as a wind generator or wind power plant.

[0006]Wind turbines are typically categorized according to the vertical or
horizontal axis about which the blades rotate. One so-called
horizontal-axis wind generator is schematically illustrated in FIG. 1 and
available from General Electric Company. This particular "up-wind"
configuration for a wind turbine 2 includes a tower 4 supporting a
nacelle 6 enclosing a drive train 8. The blades 10 are arranged on a
"spinner" or hub 9 to form a "rotor" at one end of the drive train 8
outside of the nacelle 6. The rotating blades 10 drive a gearbox 12
connected to an electrical generator 14 at the other end of the drive
train 8 arranged inside the nacelle 6 along with a control system 16 that
may receive input from an anemometer 18.

[0007]The blades 10 generate lift and capture momentum from moving air
that is them imparted to the rotor as the blades spin in the "rotor
plane." Each blade 10 is typically secured to the hub 9 at its "root"
end, and then "spans" radially "outboard" to a free, "tip" end. The
front, or "leading edge," of the blade 10 connects the forward-most
points of the blade that first contact the air. The rear, or "trailing
edge," of the blade 10 is where airflow that has been separated by the
leading edge rejoins after passing over the suction and pressure surfaces
of the blade. A "chord fine" connects the leading and trailing edges of
the blade 10 in the direction of the typical airflow across the blade and
roughly defines the plane of the blade. The length of the chord line is
simply the "chord."

[0008]Commonly-owned U.S. Pat. No. 7,458,777 is incorporated by reference
here in its entirety and discloses a wind turbine rotor assembly and
acoustic flap. FIG. 2 from that patent is a perspective view of the
turbine blade 106 in that patent for use with the wind turbine 2 shown in
FIG. 1, or any other suitable wind turbine. For example, the blade 106
may be used to modify or replace any of the blades 10 in FIG. 1.

[0009]As discussed in that patent, the blades 106 of the turbine 100 can
in some conditions produce acoustic noise in use that is undesirable in
certain installations, such as when the turbine 100 is located in close
proximity to a populated area, and particularly to residential areas.
Such problems can be compounded when multiple blades 106 are producing
noise, and when more than one turbine 100 is located in the same general
geographic area. To overcome such issues, one or more of the blades 106
includes an acoustic flap that reduces and mitigates acoustic noise to
more acceptable levels in use. Advantageously, the noise can be reduced,
using the acoustic flaps, at a lower cost than conventional, noise
reduction techniques.

[0010]The blade 106 includes a body 130 defining a leading edge 132 and a
trailing edge 134 (shown in phantom in FIG. 2). To address acoustic noise
generation issues of the blade 106 in operation, a substantially rigid
acoustic flap 136 is secured to the blade body 130 and extends outward
and away from the trailing edge 134 in a direction of arrow 138. A distal
end 140 of the acoustic flap 136 is spaced from the trailing edge 134 and
in an exemplary embodiment the distal end 140 is substantially smooth and
continuous. That is, the distal end 140 of the acoustic flap 136 does not
include serrations or saw teeth forming sharp or discontinuous edges of
the flap 136, but rather the distal end 140 of the acoustic flap 136
extends generally uniformly parallel to the trailing edge 134 of the
blade body 130 in a smooth and uninterrupted manner. Stated another way,
the contour of the distal end 140 of the acoustic flap 136 approximately
matches the contour or geometry of the blade body trailing edge 134, but
the distal end 140 of the flap 136 is spaced a predetermined distance
from the trailing edge 134 of the blade body 130 so that the flap 136
extends beyond the trailing edge 134 while maintaining approximately the
same shape and geometry of the trailing edge 134.

[0011]In one embodiment, the acoustic flap 136 is separately provided and
fabricated from the blade body 130, and in one embodiment the flap 136 is
fabricated from a thin sheet or plate of rigid material, such as metal,
fiber reinforced plastics or rigid plastic materials, and the like having
sufficient structural strength to avoid bending and deflection of the
flap 136 when the blade 106 is subjected to applied forces, such as wind
loading force and dynamic forces and vibration encountered by the blade
106 as the blade 106 is rotated. It is understood, however, that other
materials may likewise be employed in lieu of metal and plastic
materials, provided that such materials exhibit sufficient rigidity to
withstand applied forces in use when the blade 106 is used in a wind
turbine application. Thin sheet or plate materials suitable for the flaps
136 may be acquired from a variety of manufacturers at relatively low
cost, and the flaps 136 may be cut, stamped, or otherwise separated from
a larger sheet of material in a relatively simple manner with minimal
cost and machining.

[0012]FIG. 3 is a cross sectional view of the turbine blade 106 from FIG.
2 including a high pressure side 150 and a low pressure side 152
extending between the leading edge 132 and the trailing edge 134 of the
blade body 130. While the body 130 shown in FIG. 3 is hollow in cross
section, it is recognized that hollow solid bodies may alternatively be
used in another embodiment. The blade body defines a chord distance or
dimension C between the leading edge 132 and the trailing edge 134, and
the distal end 140 of the acoustic flap 136 extends outwardly and away
from the trailing edge 134 for a distance F that is a specified fraction
of the chord distance C. In an exemplary embodiment, F is about 3% or
less of the chord distance C.

[0013]Also, in an exemplary embodiment, the acoustic flap 136 has a
thickness T, measured between the major surfaces of the flap 136 that is
much less than a thickness of the blade trailing edge 134. In one
embodiment, the flap thickness T may be up to about 0.3% of the chord
distance C to achieve noise reduction without negatively impacting the
efficiency of the blades 106 to produce electricity. While exemplary
dimensions are provided, it is understood that such dimensions are for
illustrative purposes only, and that greater or lesser dimensions for T
and F may be employed in other embodiments.

[0014]The acoustic flap 136 in one embodiment is secured to an outer
surface 154 of the blade body 130 is and substantially flush with the
outer surface 154 to avoid disturbance of airflow over the pressure side
150 when the flap 136 is attached to the blade 106. In a further
embodiment, a small recess or groove (not shown) could be provided in the
blade outer surface 154 to receive the flap 136 so that an outer surface
of the flap 136 is substantially flush and continuous with the outer
surface 154 of the blade body 130. The flap 136 is secured, fixed or
bonded to the outer surface 154 with, for example, a known adhesive, tape
or other affixation methods known in the art that securely maintain the
flap 136 to the blade body outer surface 154. The flap 136 may be mounted
to the blade body 130 mechanically, chemically, or with a combination of
mechanical and chemical bonding methods. In an alternative embodiment,
the flap 136 may be integrally or monolithically formed into the blade
body 130 if desired.

[0015]The flap 136 is extended from, affixed to or secured to the blade
body 130, for example, adjacent the trailing edge 134 on one side of the
blade body 130, namely the pressure side 150 of the blade body 130 in one
exemplary embodiment. Rivets, screws or other fasteners that would
project upwardly from the outer surface 154 of the blade body 130 and
disrupt airflow across or above the blade are preferably avoided. Also,
the acoustic flap 136 is uniformly bonded to the outer surface 154 along
substantially the entire length of the blade trailing edge 134, thereby
avoiding air gaps between the flap 136 and the blade outer surface 154
that could cause the flap 136 to separate from the blade body 130, or air
gaps that could cause airflow disturbances that could impair the
efficiency of the wind turbine 2 (FIG. 1) or produce acoustic noise in
operation.

[0016]It is believed that a thin acoustic flap 136 applied to the pressure
side 150 of the trailing-edge 134 of the blade 106 can decrease noise
emission or avoid a tonality in use, and that noise reduction may be
realized using the acoustic flap 136. In particular, for blade bodies 130
having a relatively thick trailing edge 134, such as about 3 mm in an
exemplary embodiment, the acoustic flap 136 has been found to remove
negative effects of a thick trailing edge. In general, and absent the
acoustic flap 136, as the thickness of the trailing edge 134 increases,
so does the resultant acoustic noise of the blade in use. The acoustic
flap 136, however, has been found to mitigate noise when thicker trailing
edges are employed.

[0017]A generally low cost and straightforward solution to noise issues of
turbine blades in use is provided by virtue of the acoustic flap 136, and
the flap 136 may be rather easily applied and retrofitted to existing
turbine blades as desired. Additionally, if the flaps 136 are damaged,
they may be rather easily replaced. A versatile, noise reduction feature
is therefore provided that may be used in varying types of blades as
desired. The acoustic flaps 136 may be used in combination with other
known noise reducing features if desired, including but not limited to
surface treatments to the blade body, to further reduce trailing edge
noise broadband and tonality of the turbine blades in use. Considered
over a number of blades and a number of turbines, substantial noise
reduction may be achieved.

BRIEF DESCRIPTION OF THE INVENTION

[0018]These and other drawbacks associated with such conventional
approaches are addressed here in by providing, in various embodiments, a
wind turbine blade including a permeable flap extending from a trailing
edge of the blade.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]Various aspects of this technology will now be described with
reference to the following figures ("FIGs.") which are not necessarily
drawn to scale, but use the same reference numerals to designate
corresponding parts throughout each of the several views.

[0020]FIG. 1 is a schematic side view of a conventional wind generator.

[0021]FIG. 2 is a perspective view of a conventional wind turbine blade.

[0022]FIG. 3 is a cross-sectional view of the conventional turbine blade
shown in FIG. 2.

[0023]FIG. 4 is a partial orthographic view of a flap for the wind turbine
blade shown in FIGS. 2 and 3.

[0024]FIG. 5 is a partial orthographic view of another flap for the wind
turbine blade shown in FIGS. 2 and 3.

[0025]FIG. 6 is a partial orthographic view of another flap for the wind
turbine blade shown in FIGS. 2 and 3.

DETAILED DESCRIPTION OF THE INVENTION

[0026]FIGS. 4-6 illustrate various configurations for a permeable flap 200
for use with the wind turbine blade 10 shown in FIG. 1. For example, the
permeable flap 200 will extend from a trailing edge of the blade 10, and,
in this regard, may be used to replace, modify, or supplement the rigid
flap 136 shown in FIGS. 2 and 3. The permeable flap 200 may be configured
similar to the flap 136 described above with regard to FIGS. 2 and 3
and/or in other configurations. For example, the permeable flap 200 may
also be porous and/or flexible, and/or the permeable flap 200 may be
integrated with the blade 10 or a portion of the blade 10. The permeable
flap 200 may extend continuously or intermittently along some or all of
the span of the blade 10. Furthermore, the flap 200 may be applied to
either the pressure or suction side of the blade 10.

[0027]As illustrated in FIG. 4, the permeable flap 200 may include a
perforated surface. The perforations 202 may include cylindrical holes
and/or holes of other shapes, such as slits or slots. The perforations
202 may be microscopic in size, or otherwise too small to be seen by the
unaided eye. Non-permeable sheet materials with regular perforations 202
through the material (such as slitted or perforated sheets) in order to
provide permeability are expected to produce adequate noise reduction
when surface porosities are less than about 20% of the surface area of
the permeable flap 200. It is also expected that many smaller
perforations 202 in the form of holes and/or slits through an otherwise
non-permeable flap 200 will produce better results than fewer large holes
spread over the same percentage of surface area of the flap. Increasing,
or otherwise varying, the surface porosity and corresponding permeability
of the flap 200 in direction of flow over the flap is also expected to
provide better results. For example, in the case of an otherwise
non-permeable flap, providing a higher density of perforations 202 near
the trailing edge of the flap 200 is expected to offer improved results.

[0028]As illustrated in FIG. 5, wherein the permeable flap 200 may include
one or more felt surfaces 204. Other permeable textiles may also be used
including animal textiles such as wool or silk, plant textiles, mineral
textiles and glass, basalt and/or asbestos fibers, and synthetic textiles
such as GORE-TEX® membranes and fabrics, polyester, acrylics, nylon,
spandex, Kevlar® and/or any combination of these and textiles.
Although FIG. 5 illustrates equally-spaced felt strips that cover only a
portion of the permeable flap 200, the felt 204 may also completely cover
the permeable flap 200. For example, the felt 204 may be used to cover an
otherwise open support structure. Felt may also be used to cover the
openings of the perforations 202 and/or perforations 202 may also be
provided in the felt material for additional permeability.

[0029]As illustrated in FIG. 6, the permeable flap 200 may also include a
screen 206, such as a sintered or unsintered wire mesh screen. The screen
206 may also be formed from other fibers, including textile fibers. The
screen may also act as an underlying structure for supporting a textile
such as felt and/or as a protective layer over the felt 204. For example,
highly flexible material such as felt, Kevlar®, and fabrics may be
applied over a more rigid framework or underlying structure while more
rigid materials such as perforated plate, stiff sintered screen, or slits
may be used without additional support structure and/or as a base for the
flexible material.

[0030]The flap 200 may be permeable over its entire length and width, or
just a portion thereof, and the permeability may change over any
dimension of the flap. The permeable flap 200 may also be arranged in any
configuration. For example, the permeable flap 200 may extend (a distance
"F" in FIG. 3) from a trailing edge of the blade 10 (FIG. 1) between
approximately 1% and 5% of a chord of the blade, between approximately 2%
and 4% of a chord of the blade, or about 3% of a chord of the blade. The
permeable flap may also have a thickness ("T" in FIG. 3), less than about
0.5% of a chord of the blade, or less than about 0.3% of a chord of the
blade. For example, the thickness "T" may be around 1-2 mm (or 0.1-0.2%
of chord) along some or all of the span of the flap 200. In that case,
since the chord changes along the span, the dimension "T" as a percentage
of chord will be closer to 0.5% near the tip and closer 0.1% or less near
the inboard portion of the flap 200. Furthermore, for a substantially
stiff material such as perforated sheet metal or fiberglass, the
dimension "T" may be much smaller.

[0031]The technology described above offers various advantages over
conventional approaches by reducing wind turbine blade trailing edge
noise at low cost and with minimal performance impact. For example, the
permeability of the flap 200 allows communication of the pressure field
between the pressure and suction sides of the blade 10 in order to
improve the noise reduction capabilities of the conventional flap 136.
Similarly, flexibility in the permeable flap 200 allows the flap to adapt
to flow conditions by changing shape. For a flexible permeable flap 200,
the pressure difference between the upper and lower surfaces of the blade
will cause the mean shape of the flap to adapt in a compliant manner in a
way that reduces the trailing edge vortex strength and reduces noise. The
shape of the resulting flap then would be controlled by the material
flexibility and permeability of the flap material. Lower values of
surface porosity (down to 0% percent open area) and corresponding
permeability will generally allow less pressure relief between pressure
and suction sides of the blade, but more bending in the flap. Higher
values of surface porosity (up to about 50% percent open area) and
corresponding permeability will generally allow more pressure relief, but
less change in the shape of the acoustic flap due to pressure
differential between the upper and lower surfaces. The permeability
and/or flexibility of the flap 200 may be adjusted with different
materials and/or perforation densities in order to affect the noise
source characteristics and sound radiation efficiency of a particular
blade 10 for various blade configurations and/or operating environments.

[0032]It should be emphasized that the embodiments described above, and
particularly any "preferred" embodiments, are merely examples of various
implementations that have been set forth here to provide a clear
understanding of various aspects of this technology. One of ordinary
skill will be able to alter many of these embodiments without
substantially departing from scope of protection defined solely by the
proper construction of the following claims.